The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon.
1. Field of the Invention
This invention relates generally to a semipermeable membrane, and the various uses therefor. The invention relates more specifically to a semipermeable membrane comprising a self-assembled sulfonated block copolymer comprising poly (styrene-co-styrene sulfonic acid) segments and polyisobutylene segments.
2. Description of the Related Art
Various permeable materials having a wide range of mechanical and transport properties are known. Depending upon the particular application in which the permeable material is to be employed, however, certain combinations of properties are required.
For example, in a protective apparel application, it may be desirable that the material transport air and water vapor while blocking the transport of harmful chemicals, and be lightweight and flexible over a broad temperature range.
The U.S. Army has multiple requirements for chemical and biological protective materials. Butyl rubber is often used in standard protective clothing. However, garments made from butyl rubber are bulky and nearly impermeable to air and moisture (I. Lee, Yang and Wilusz; Polymer Engineering and Science, 1996, 36, 1217), resulting in unbearable levels of heat inside the garments during use.
In an apparel application, therefore, a need exists for a material that provides the requisite level of protection, but that is also semipermeable, i.e., one that allows air and moisture to permeate the garments to the extent necessary to afford comfort to the wearer.
Various references describe semipermeable materials produced from a variety of polymers. For example, U.S. Pat. No. 4,305,823 describes polymers from the group consisting of cellulose and its ester and ether derivatives, of polyvinyl alcohol and its ester derivatives, copolymers with vinyl esters and homopolymers and copolymers of acrylates and methacrylates modified with monofunctional or difunctional cyclic xcex2-ketonic acids or their esters. The modified polymers are disclosed as being suitable, however, for osmotic separation methods.
U.S. Pat. No. 4,686,137 describes polymeric materials characterized by hard segments and copolymer soft blocks comprising hydrophobic and hydrophilic components. There is no mention, however, of blocking chemicals/liquids/penetrants other than water. Similarly, U.S. Pat. No. 5,120,813 discloses simply the selective permeation of water vapor over liquid water.
U.S. Pat. No. 5,480,953 discloses membranes, but with a chemistry based primarily on polyalkylene oxides and acrylic or methacrylic ester.
U.S. Pat. No. 6,110,616 discloses the synthesis of statistical styrene-hyrogenated butadiene copolymers (as opposed to block copolymers) to make sulfonated membranes, and the application of these materials into fuel cells, and requires greater than 30% sulfonation of the styrene components.
U.S. Pat. No. 5,679,482 discloses sulfonated block copolymers of polystyrene and hydrogenated ethylene-butadiene. The base polymer, SEBS, is sulfonated to a level of greater than 25 mol %.
Therefore, although different approaches have been taken to address the need for different permeable materials in different technologies, no single material has emerged which satisfies all of the technical requirements and which presents a cost-effective alternative. More specifically, none of the aforementioned references provides a semipermeable membrane that exhibits the necessary combination of desirable properties so as to render it suitable for use in a wide variety of applications.
It is an object of the present invention to provide a semipermeable membrane that displays a combination of mechanical properties, low temperature flexibility, selective transport, ease of processability, and cost-effectiveness, so as to render it suitable for use in a wide variety of applications.
Accordingly, the present invention relates to a semipermeable membrane comprising sulfonated forms of polystyrene (xe2x80x9cPSxe2x80x9d) and polyisobutylene (xe2x80x9cPIBxe2x80x9d) base block copolymers. A major component of the copolymer is polyisobutylene, which lends low temperature flexibility to the material, and good barrier properties. Polystyrene forms the glassy component, which aggregates within the PIB matrix, thereby lending mechanical integrity to the copolymer.
The thermodynamic immiscibility of the two components results in spontaneous microphase separation. The PS block is modified such that the PS microphase provides a channel for water transport, while the PIB matrix acts as a chemical barrier.
Modification of the PS blocks is performed by conversion of the styrene monomer to styrene-sulfonic acid. The sulfonated block copolymer exhibits a hydrophilic character, and is capable of selectively transporting water. Sulfonation of the polymer does not adversely effect the material""s flexibility over a broad temperature range, or its solubility in common solvents, which makes processing economically feasible.
Accordingly, in a first embodiment, the present invention relates to a semipermeable membrane produced from an unsulfonated block copolymer comprising polystyrene and polyisobutylene segments, in which the semipermeable membrane comprises a sulfonated block copolymer comprising poly (styrene-co-styrene sulfonic acid) segments and polyisobutylene segments. Prior to modification by sulfonation, the unsulfonated polystyrene and polyisobutylene block copolymer comprises polystyrene monomer in an amount of from 5% to 90% by weight of the total unsulfonated block copolymer. After modification, from 1 to 70 mole % of the polystyrene monomer in the unsulfonated polystyrene and polyisobutylene block copolymer is converted to styrene sulfonic acid in the sulfonated block copolymer.
In another embodiment of the invention, the semipermeable membrane can be employed in a semipermeable laminate. The laminate comprises (a) one or a plurality of semipermeable membrane layers produced from an unsulfonated block copolymer comprising polystyrene and polyisobutylene segments, with the membrane layer comprising a sulfonated block copolymer comprising poly (styrene-co-styrene sulfonic acid) segments and polyisobutylene segments, wherein the unsulfonated polystyrene and polyisobutylene block copolymer comprises polystyrene monomer in an amount of from 5% to 90% by weight of the total unsulfonated block copolymer, and wherein from 1 to 70 mole % of the polystyrene monomer in the unsulfonated polystyrene and polyisobutylene block copolymer is converted to styrene sulfonic acid in the sulfonated block copolymer; and (b) a substrate, such as, for example, a fabric.
In still another embodiment, the invention can be employed as a semipermeable layer. The semipermeable layer comprises the aforementioned semipermeable membrane, and a substrate, wherein the membrane is dispersed throughout the substrate.
Finally, by virtue of the semipermeable membrane""s excellent transport and mechanical properties, and ease of processing, the various embodiments of the membrane advantageously find numerous uses, including in chemical and/or biological protective apparel, as an ion-conducting membrane for use in a fuel cell, and in medical equipment and apparel.